The present invention relates to tunable microwave dielectric monolithic integrated circuits. The invention also relates to a method for tuning the phase velocity of microwaves in a microwave monolithic integrated circuit. Tunable microwave devices as such are of considerable interest for example within microwave communication, radiosystems and cellular communications systems etc.
A number of tunable microwave devices have been suggested. U.S. Pat. No. 5,285,067 for example shows a superconducting resonator on a non-ferroelectric (linear) substrate wherein input and output respectively are formed by microstrips. Via optical illumination the properties of the superconducting films are changed (tuning) which results in a shift in resonant frequency. Apart from optical illumination also other means can be used to change or control the properties of the superconducting films and thus provide controllability. However, for optical tuning a high optical power is required and the tuning is not very effective.
U.S. Pat. No. 5,179,074 illustrates dielectric resonators in super-conducting cavities having a low loss at high microwave power levels. However, the designs are bulky and involve a complicated and expensive fabrication technology and they are not suitable for monolithic microwave integrated circuits.
From WO 94/13028 a number of tunable microwave devices based on high temperature superconductors and ferroelectric thin film microstrip waveguide designs are known. However, these devices suffer from unacceptable high microwave losses and low tunability due to the low inherent quality of the ferroelectric film. Moreover the microwave power handling capability is low among other reasons due to the low quality of the ferroelectric film and the high non-linear behaviour (over-tone generation) of narrow HTS-strips.
Furthermore, image waveguides comprising a dielectric arranged on top of a metallic ground plane have been used for millimeter and submillimeter wavelength integrated circuits, see for example P. Bhartia and I. J. Bahl in "Millimeter Wave Engineering and Applications", J. Wiley, 1984 and for devices in the optical spectrum, c.f. M. J. Adams, "An Introduction to Optical Waveguides", J. Wiley, 1981. However, the implementation of this Microwave Integrated circuit (MIC) technology at frequencies below 3 GHz has been limited by dielectrics having a low dielectric constant, and low losses, tan.delta.&gt;10.sup.-4, which imply large dimensions of the dielectric MIC.
Generally, dielectric materials used in microwave technology have had a dielectric constant of 0-100, which would only result in gigantic devices at the frequencies of about 1-2 GHz. In "High Temperature Superconducting Microwave Devices", by Z-Y Shen, Artech House, 1994 dielectric resonators based on TM.sub.01.delta. delta modes are disclosed. The dielectric resonator is clamped between thin high temperature superconducting films which are deposited on separate substrates arranged between the thin film and the dielectric. Even if the surface resistance and the associating microwave losses of the high temperature superconductor materials are extremely low at 1-2 GHz, typically 10.sup.-4 Ohm, these devices suffer from not having the desirable properties in that the dimensions of the superconducting films and the dielectric substrates at these frequencies (e.g. 1-2 GHz) are large and the devices are expensive to fabricate. Moreover they can only be tuned mechanically and therefore the devices get bulky and introduce complex problems in connection with vibrations or microphonics.